dep(dev): pin psych to v4 until v5 builds in CI

backport of 6a3254c

Author: flavorjones

nokogiri.gemspec

@@ -325,6 +325,7 @@ Gem::Specification.new do |spec|
   spec.add_development_dependency("hoe-markdown", "~> 1.4")
   spec.add_development_dependency("minitest", "~> 5.15")
   spec.add_development_dependency("minitest-reporters", "~> 1.4")
+  spec.add_development_dependency("psych", "~> 4.0") # psych 5 isn't building in places yet https://github.com/ruby/setup-ruby/issues/409
   spec.add_development_dependency("rake", "~> 13.0")
   spec.add_development_dependency("rake-compiler", "= 1.1.7")
   spec.add_development_dependency("rake-compiler-dock", "= 1.2.2")